Reconstructing Transaction Order Using Clump Tags

ABSTRACT

A method and system for enforcing ordering rules for transactions are presented. The method and system generates transaction clump tags for each transaction before the transactions are stored in various type specific transaction queues. A transaction clump tag decoding unit decodes the transaction clump tag to recover temporal information regarding the transaction to avoid violations of the ordering rules.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/855,192, entitled “Reconstructing Transaction Order Using Clump Tags”filed May 26, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data systems. More specifically, thepresent invention relates to processing data transactions withoutviolating ordering rules.

2. Discussion of Related Art

Modern data systems typically follow a producer-consumer model. FIG. 1shows a data system 100 having a consumer 110, a data storage device120, a bus 130, a bridge 140, a bus 150, a producer 160, a flag register170, and a status register 180. In a producer-consumer model, producer160 produces data and consumer 110 consumes (i.e., uses) the data.Specifically, when requested, producer 160 writes data into data storagedevice 120 through bus 150, bridge 140, and bus 130. After writing datainto data storage device 120, producer 160 sets a flag in flag register170 to indicate that the data has been written into data storage device120. Consumer 110 waits until the flag is set in flag register 170before using the data in data storage device 120. Consumer 110 alsoresets the flag in flag register 170 and writes a completion status codein status register 180 to indicate that consumer 110 has consumed thedata. FIG. 1 shows one example of the producer-consumer model. In actualimplementation, each component (i.e., consumer, producer, data storagedevice, flag register, and status register) of the producer-consumermodel can be placed on either side of the bridge.

Data systems generally support multiple transaction types, such asposted transactions (e.g., data writes not requiring a response),non-posted transactions (e.g., data read requests), and completiontransactions (e.g., data in response to data read requests. To handlethe various transaction types, a transaction queue for each transactiontype can be used. FIG. 2( a) shows a series of transactions 200, whichincludes (in order) a posted transaction P_01, a non-posted transactionNP_02, a completion transaction C_03, a posted transaction P_04, anon-posted transaction NP_05, a non-posted transaction NP_06, a postedtransaction P_07, a completion transaction C_08, a posted transactionP_09, and a posted transaction P_10.

As illustrated in FIG. 2( b), the transactions of FIG. 2 are placed intotype specific transaction queues in the order the transactions arereceived, such as, a posted transaction queue 210, a non-postedtransaction queue 220, and a completion transaction queue 230.Specifically, posted transactions P_01, P_04, P_07, P_09, and P_10 areplaced in posted transaction queue 210; non-posted transactions NP_02,NP_05, and NP_06 are placed in non-posted transaction queue 220; andcompletion transactions C_03 and C_08 are placed in completiontransaction queue 230. Transactions of the same type are processed inthe order received. However, transactions of different type are allowedto pass previously issued transaction of another type under certainordering rules. For example, in one interpretation of the ordering rulesof a PCI or PCI Express bus, posted transaction are allowed to passpreviously received non-posted transactions and previously receivedcompletion transactions. Non-posted transactions are allowed to passpreviously received completion instructions but not allowed to passpreviously received posted transactions. Completion instructions areallowed to pass previously received non-posted transaction but notallowed to pass previously received posted instructions. These orderingrules are to prevent a consumer (in the producer-consumer model) fromconsuming data before a producer is finished and to prevent a producerfrom producing data before a consumer is finished. These ordering rulesare summarized in Table 1. Table 1 indicates whether the transactiontype in the first column may pass the previously issued transaction ofthe transaction types in the following columns.

TABLE 1 Transaction Posted Non-Posted Completion POSTED NO YES YESNON-POSTED NO NO YES COMPLETION NO YES YES

Improved bandwidth is achieved when some transaction types are blockedfor whatever reason but another transaction type can proceed withoutviolating the ordering rules. For example, after posted transaction P_01completes, if non-posted transaction NP_02 is blocked, completiontransaction C_03 and posted transaction P_04 are allowed to proceedahead of non-posted transaction NP_02. Thus, a blocked transaction doesnot necessarily halt usage of the data system. However, due to theordering rules, blocked posted transactions may halt usage of the datasystem until the block is resolved.

For example, if posted transaction P_01 is blocked, no other transactioncan be processed because neither non-posted transactions nor completiontransactions are allowed to pass a previously received postedtransaction. However, if posted transaction P_04 is blocked (afterposted transaction P_01 has already been processed), non-postedtransaction NP_02 and completion transaction C_03 can proceed. If postedtransaction P_04 is still blocked after non-posted transaction NP_02 hasbeen processed, non-posted transaction NP_05 is blocked by the orderingrules, because non-posted transaction NP_05 is not allowed to pass thepreviously received posted instruction P_04.

Thus, when processing the transaction, the information relating to theoriginal order that the transactions were received is needed to followthe ordering rules. However, once the transactions have been separatedinto the various type-specific transaction queues, the original order ofthe transaction cannot be reconstructed from the transactions queues.However, the order of the transactions within each transaction type ispreserved in the transaction queue. Conventionally, a transactionsequence number has been added to each transaction as the transaction isstored in the transaction queues. However, the transaction sequencenumber increases the size of the transaction queue by approximately 10%.Alternatively, an additional memory may be used to store the transactionsequence numbers. However, the additional memory increases thecomplexity of the data system.

Hence, there is a need for a method or system that can processtransactions in the transaction queues in accordance with the orderingrules without requiring the increased queue size or additionalcomplexity of using a transaction sequence number.

SUMMARY

Accordingly, the present invention provides a method and system fortagging transactions with a transaction clump tag as they are placed inthe transaction queues and a method and system for decoding thetransaction clump tags while processing the transaction to insure theordering rules of the data system are not violated.

For example, in one embodiment of the present invention a transactionclump tagging unit embeds transaction clump tags in the datatransactions as the data transactions are sorted into transaction typequeues. Specifically, the transaction clump tagging unit assigns a firsttransaction clump tag having a second-transaction-type bit to a firstdata transaction of a first transaction type. The first data transactionis stored in a first transaction type queue. The transaction clumptagging unit also assigns a second transaction clump tag to a seconddata transaction of a second transaction type. The second transactionclump tag includes a second-transaction-type bit set to the inverse ofthe second-transaction-type bit of the first transaction clump tag. Thetransaction clump tagging unit also assigns a third transaction clumptag to a third data transaction of the first transaction type. The thirdtransaction clump tag includes a second-type-transaction bit set equalto the inverse of the second-type-transaction bit of the secondtransaction clump tag.

For PCI systems and PCI express systems, the first transaction type canbe posted transactions and the second transaction type can be non-postedtransactions or completion transactions. Therefore, the transactionclump tagging unit assigns a first transaction clump tag having anon-posted bit to a first posted transaction. The first postedtransaction is stored in a posted transaction queue. The transactionclump tagging unit also assigns a second transaction clump tag to anon-posted transaction. The second transaction clump tag includes anon-posted bit set to the inverse of the non-posted bit of the firsttransaction clump tag. The transaction clump tagging unit also assigns athird transaction clump tag to a second posted transaction. The thirdtransaction clump tag includes a non-posted bit set equal to the inverseof the non-posted bit of the second transaction clump tag.

A transaction clump tag decoding unit decodes the transaction clump tagto insure the ordering rules are followed while processing the datatransactions. Specifically, in one embodiment of the present inventionthe transaction clump tag decoding unit includes a counter that isincremented when the second transaction-type bit of a transaction clumptag of a previous first type data transaction is not equal to the secondtransaction-type bit of a transaction clump tag of a current first typedata transaction. The counter is decremented when asecond-transaction-type bit of a transaction clump tag of a previoussecond type data transaction is not equal to the second-transaction-typebit of a transaction clump tag of a current second type datatransaction. The value of the counter determines whether the currentsecond type data transaction should be blocked.

The present invention will be more fully understood in view of thefollowing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a model of a conventional data system.

FIG. 2( a) is an illustration of a sequence of transactions.

FIG. 2( b) is a set of transaction queues used to store a sequence oftransactions.

FIG. 3 is an illustration of transaction clumps in accordance with oneembodiment of the present invention.

FIG. 4 is a block diagram of a transaction ordering system in accordancewith one embodiment of the present invention.

FIG. 5 is an illustration of transaction clumps with transaction clumptags in accordance with one embodiment of the present invention.

FIG. 6 is block diagram transaction processing system in accordance withone embodiment of the present invention.

FIG. 7 is block diagram of a transaction clump tag unit in accordancewith the present invention.

DETAILED DESCRIPTION

As explained above, a sequence of transactions is divided into aplurality of transaction queues based on the types of transactions.However, when the transactions are processed the transaction orderingrules must be followed. In accordance with the present invention, asequence of transactions is conceptually organized into differentclumps. The transaction types within each clump can be processed in anyorder. However, transaction of the same type within a clump must beprocessed in the order that the transaction were received. Becausetransactions of the same type are stored in the same transaction queue,the order of transactions of each type is maintained by the structure ofthe transaction queues.

As shown in Table 1 above, no transaction can pass a previously receivedposted transaction. Therefore, posted transactions are used to definethe end boundary of the transaction clumps. However, in most embodimentsof the present invention groups of consecutive posted transaction areplaced in the same clump. Because non-posted transaction can passcompletion transactions and completion transactions can pass non-postedtransaction, all non-posted transaction and completion transactionbetween two posted transaction are placed in the same clump as theposted transaction following the non-posted and completion transactions.FIG. 3 illustrates how sequence of transactions 200 is divided intotransaction clumps 310, 320, 330, and 340, in accordance with oneembodiment of the present invention. Specifically, transaction clump 310includes only posted transaction P_01 because there were no non-postedtransactions or completion transactions preceding posted transactionP_01. Transaction clump 320 includes non-posted transaction NP_02,completion transaction C_03, and posted transaction P_04. Transactionclump 330 includes non-posted transactions NP_05 and NP_06 and postedtransaction P_07. Transaction clump 340 includes completion transactionC_08 and posted transaction P_09 and P_10. Other embodiments of thepresent invention may place posted transaction P_10 in a separatetransaction clump.

As explained above, transactions are assigned into the same transactionclump so that the ordering rules allow any transaction in the clump tobe processed ahead of any other transaction in the transaction clump ofa different transaction type. For example in transaction clump 320,posted transaction P_04 can be processed before non-posted transactionNP_02 and completion transaction C_03 because the ordering rules allowsa posted transaction to pass previously received non-posted transactionand to pass previously received completion transaction. Non-postedtransaction NP_02 can be processed before posted transaction P_04 andcompletion transaction C_03 because non-posted transaction NP_02 wasreceived before both posted transaction P_04 and completion transactionC_03. Furthermore, even if completion transaction C_03 was receivedbefore non-posted transaction NP_02, non-posted transaction NP_02 couldbe processed before completion transaction C_03 because the orderingrules allows a non-posted transaction to pass previously receivedcompletion transactions. Completion transaction C_03 can be processedbefore posted transaction P_04 because completion transaction C_03 wasreceived before posted transaction P_04. Completion transaction C_03 canbe processed before non-posted transaction NP_02 because the orderingrules allows a completion transaction to pass a previously receivednon-posted transaction. Posted transaction P_01 is not included intransaction clump 320 because non-posted transaction NP_02 andcompletion transaction C_03 cannot be processed before postedtransaction P_01 because the ordering rules forbid non-postedtransaction and completion transactions from passing previously receivedposted transaction. Non-posted transaction NP_05 is excluded fromtransaction clump 320 because the non-posted transaction NP_05 cannot beprocessed before posted transaction P_04.

In transaction clump 330, posted transaction P_07 can be processedbefore non-posted transactions NP_05 and NP_06 because the orderingrules allow a posted transaction to pass previously received non-postedtransactions. Because non-posted transaction NP_05 and NP_06 werereceived before posted transaction P_07, non-posted transactions NP_05and NP_06 can be processed before posted transaction P_07. With strictlyordered queues, non-posted transaction NP_05 is processed beforenon-posted transaction NP_06. However, some embodiments of the presentinvention may use buffers that allow out of order processing ofnon-posted transactions.

In transaction clump 340, posted transactions P_09 and P_10 can beprocessed before completion transaction C_08 because the ordering rulesallow a posted transaction to pass previously received completiontransactions. However, posted transaction P_09 is processed beforeposted transaction P_10 based on the interpretation of the orderingrules given in Table 1. However, in some embodiments of the presentinvention, posted transactions are allowed to pass previously receivedposted transaction under “relaxed” ordering rules. Because completiontransaction C_08 was received before posted transactions P_09 and P_10,completion transaction C_08 can be processed before posted transactionsP_09 and P_10.

Once the transaction are placed into the appropriate transaction clumps,determining whether a transaction is blocked by the ordering rules isrelatively straightforward. Specifically, if a posted transaction of apreceding transaction clump has not been processed, transactions in afollowing transaction clump are blocked.

A simple way to track transaction clumps would be to assign a sequentialtransaction clump identification to each transaction clump. As eachtransaction is entered into a transaction queue the transaction clumpidentification is stored with each transaction. If the transaction clumpidentification of a non-posted transaction is less than or equal to thetransaction clump identification of the oldest unprocessed postedtransaction in the posted transaction queue or if the posted transactionqueue is empty, then the non-posted transaction is not blocked by theordering rules. Similarly, if the transaction clump identification of acompletion transaction is less than or equal to the transaction clumpidentification of the oldest unprocessed posted transaction in theposted transaction queue or if the posted transaction queue is empty,then the completion transaction is not blocked by the ordering rules.However, while the number of transaction clumps is less than the numberof transactions, the number of bits required for the transaction clumpidentification is not significantly less than the number of bitsrequired for a sequential transaction identification. Therefore, otherembodiments of the present invention use a small transaction clump tagthat is stored with or in each transaction. The transaction clump tagscan be decoded to determine whether a transaction is blocked by theordering rules.

FIG. 4 shows a block diagram of a transaction clump tagging and decodingsystem 400 in accordance with one embodiment of the present invention.Transaction clump tagging and decoding system 400 includes a transactionclump tagging unit 410, a posted transaction queue 420, a non-postedtransaction queue 430, a completion transaction queue 440, and atransaction clump tag decoding unit 450. Transaction clump tagging unit410 receives incoming transaction, generates a clump tag for eachtransaction and places the transaction and the transaction clump taginto the appropriate transaction queue. In some embodiments of thepresent invention, the clump tags are embedded in unused (i.e.,reserved) bits of the transactions. An embodiment of transaction clumptagging unit 410 is described below. Posted transaction queue 420 storesincoming posted transactions, non-posted transaction queue 430 storesnon-posted transactions, and completion transaction queue 440 storescompletion transactions. Each transaction queue works like afirst-in-first-out (FIFO) buffer. Posted transaction queue 420 providesa posted transaction queue empty flag PTQ_EF to transaction clump tagdecoding unit 450. Posted transaction queue empty flag is driven to anempty logic level (e.g., logic high) when posted transaction queue 420is empty and driven to a non-empty logic level (e.g., logic low) whenposted transaction queue 420 contains a posted transaction.

Transaction clump tag decoding unit 450 buffers the current (i.e.,oldest unprocessed) transaction from each transaction queue and decodesthe transaction clump tags from the current transactions to determinewhich transactions can be processed without violating the orderingrules. Thus, transaction clump tag decoding unit 450 provides currentposted transaction CPT, current non-posted transaction CNPT, currentcompletion transaction CCT, a current non-posted transaction blockedsignal CNPT_BS, and a current completion transaction blocked signalCCT_BS. Current non-posted transaction blocked signal CNPT_BS indicateswhether the current non-posted transaction is blocked by the orderingrules. Specifically, current non-posted transaction blocked signalCNPT_BS is driven to a blocked state (typically logic high) when thecurrent non-posted transaction is blocked by the ordering rules anddriven to a not blocked state (typically logic low) when the currentnon-posted transaction is not blocked by the ordering rules. Similarly,current completion transaction blocked signal CCT_BS indicates whetherthe current completion transaction is blocked by the ordering rules. Inembodiments of the present invention that embed transaction clump tagsin the transactions, transaction clump tag decoding unit 450 alsomodifies the current transaction to remove the transaction clump tags.An embodiment of transaction clump tag decoding unit 450 is describedbelow.

FIG. 5 illustrates the generation of transaction clump tags inaccordance with one embodiment of the present invention. Specifically,FIG. 5 shows the transactions from FIG. 3 with the correspondingtransaction clump tags (TCT) for each transaction. For postedtransactions, a 2 bit transaction clump tag [b2,b1] is used, in whichthe first bit b1 is a completion bit and the second bit b2 is anon-posted bit. For non-posted transaction and completion transactions,only a single bit transaction clump tag is required. However forconsistency, the embodiments of FIGS. 4, 5, 6, and 7 use two-bittransaction clump tags, having a non-posted bit (the second bit) and acompletion bit (the first bit), for all transactions. Thus, in FIG. 5,transaction clump tags for non-posted transactions only make use of thenon-posted bit and the completion bit is shown with an “X” signifying adon't care condition (i.e., the completion bit can be logic 0 or logic 1without changing the behavior of the system). Conversely, transactionclump tags for completion transactions only make use of the completionbit and the non-posted bit is shown with an “X”.

For clarity, the embodiment of FIG. 5 defines the transaction clump tagof the first posted transaction to be [1,1]. One skilled in the art canadapt the principles described herein to other defined initial values.Thus, posted transaction P_01 is shown with a transaction clump tagequal to [1,1]. If posted transaction P_01 were preceded by a non-postedtransaction, the transaction clump tag of the non-posted transactionwould be equal to [1,X]. Similarly, if posted transaction P_01 werepreceded by a completion transaction, the transaction clump tag of thenon-posted transaction would be equal to [X,1].

After the transaction clump tags of the transactions in the initialclump (i.e., transaction clump 310) are defined as explained above, thetransaction clump tags of following clumps follow a specific set ofrules. Specifically, the non-posted bit of the transaction clump tag ofa posted transaction is equal to the non-posted bit of the transactionclump tag of the most recent non-posted transaction. Similarly, thecompletion bit of the transaction clump tag of a posted transaction isequal to the completion bit of the transaction clump tag of the mostrecent completion transaction. The non-posted bit of the transactionclump tag of a non-posted transaction is equal to the inverse of thenon-posted bit of the transaction clump tag of the most recent postedtransaction. Similarly, the completion bit of the transaction clump tagof a completion transaction is equal to the inverse of the completionbit of the transaction clump tag of the most recent posted transaction.As explained above the completion bit of the transaction clump tag of anon-posted transaction and the non-posted bit of a completiontransaction are irrelevant and marked with “X”, which signifies a “don'tcare” condition.

Applying the rules above, the non-posted bit of the transaction clumptag for non-posted transaction NP_02 is equal to the inverse of thenon-posted bit of the transaction clump tag of the most recent postedtransaction, which is posted transaction P_01. The completion bit of thetransaction clump tag for non-posted transaction NP_02 is irrelevant.Thus the transaction clump tag of non-posted transaction NP_02 is equalto [0,X]. The non-posted bit of the transaction clump tag of completiontransaction C_03 is irrelevant. The completion bit of the transactionclump tag of completion transaction C_03 is equal to the inverse of thecompletion bit of the transaction clump tag of the most recent postedtransaction, which is posted transaction P_01. Thus, the transactionclump tag of completion transaction C_03 is equal to [X,0]. Thenon-posted bit of the transaction clump tag of posted transaction P_04is equal to the non-posted bit of the transaction clump tag of the mostrecent non-posted transaction, which is non-posted transaction NP_02.The completion bit of the transaction clump tag of posted transactionP_04 is equal to the completion bit of the transaction clump tag of themost recent completion transaction, which is completion transactionC_03. Thus, the transaction clump tag of posted transaction P_04 isequal to [0,0].

The non-posted bit of the transaction clump tag for non-postedtransaction NP_05 is equal to the inverse of the non-posted bit of thetransaction clump tag of posted transaction P_04. The completion bit ofthe transaction clump tag for non-posted transaction NP_05 isirrelevant. Thus, the transaction clump tag of non-posted transactionNP_05 is equal to [1,X]. The non-posted bit of the transaction clump tagfor non-posted transaction NP_06 is equal to the inverse of thenon-posted bit of the transaction clump tag of posted transaction P_04.The completion bit of the transaction clump tag for non-postedtransaction NP_06 is irrelevant. Thus, the transaction clump tag ofnon-posted transaction NP_06 is equal to [1,X]. As illustrated by thetransaction clump tags of non-posted transactions NP_05 and NP_06, thetransaction clump tags of all transactions of the same type within atransaction clump are the same. The non-posted bit of the transactionclump tag of posted transaction P_07 is equal to the non-posted bit ofthe transaction clump tag of non-posted transaction NP_06. Thecompletion bit of the transaction clump tag of posted transaction P_07is equal to the completion bit of the transaction clump tag ofcompletion transaction C_03. Thus, the transaction clump tag of postedtransaction P_07 is equal to [1,0].

The completion bit of the transaction clump tag for completiontransaction C_08 is equal to the inverse of the completion bit of thetransaction clump tag of posted transaction P_07. The non-posted bit ofthe transaction clump tag for completion transaction C_08 is irrelevant.Thus, the transaction clump tag of completion transaction C_08 is equalto [X,1]. The non-posted bit of the transaction clump tag of postedtransaction P_09 is equal to the non-posted bit of the transaction clumptag of non-posted transaction NP_06. The completion bit of thetransaction clump tag of posted transaction P_09 is equal to thecompletion bit of the transaction clump tag of completion transactionC_08. Thus, the transaction clump tag of posted transaction P_09 isequal to [1,1]. The non-posted bit of the transaction clump tag ofposted transaction P_10 is equal to the non-posted bit of thetransaction clump tag of non-posted transaction NP_06. The completionbit of the transaction clump tag of posted transaction P_10 is equal tothe completion bit of the transaction clump tag of completiontransaction C_08. Thus, the transaction clump tag of posted transactionP_10 is equal to [1,1].

FIG. 6 is a circuit that can be used to generate the transaction clumptags. The embodiment of FIG. 6 includes a posted clump tag non-postedbit register 610, an inverter 615, a posted clump tag completion bitregister 620, an inverter 625, a non-posted clump tag non-posted bitregister 630, and a completion clump tag completion bit register 640.Posted clump tag non-posted bit register 610 and posted clump tagcompletion bit register 620 contain the non-posted bit and completionbit, respectively, of the transaction clump tag of the most recentposted transaction. Non-posted clump tag non-posted bit register 630contains the non-posted bit of the transaction clump tag of the mostrecent non-posted transaction. Completion clump tag completion bitregister 640 contains the completion bit of the transaction clump tag ofthe most recent completion transaction. Posted clump tag non-posted bitregister 610 and posted clump tag completion bit register 620 areinitialized to logic 0. Non-posted clump tag non-posted bit register 630and completion clump tag completion bit register 640 are initialized tologic 1.

When a new posted transaction is received, posted clump tag non-postedbit register 610 is updated with the bit value from non-posted clump tagnon-posted bit register 630 and posted clump tag completion bit register620 is updated with the bit value from completion clump tag completionbit register 640. The updated values in posted clump tag non-posted bitregister 610 and posted clump tag completion bit register 620 are usedas the non-posted bit and completion bit of the transaction clump tag ofthe new posted transaction. When a new non-posted transaction isreceived, non-posted clump tag non-posted bit register 630 is updatedwith the value from posted clump tag non-posted bit register 610inverted through inverter 615. The updated value in non-posted clump tagnon-posted bit register 630 is used as the non-posted bit of thetransaction clump tag of the new non-posted transaction. When a newcompletion transaction is received, completion clump tag completion bitregister 640 is updated with the value from posted clump tag completionbit register 620 inverted through inverter 625. The updated value incompletion clump tag completion bit register 640 is used as thecompletion bit of the transaction clump tag of the new completiontransaction.

A careful review of the transaction clump tag reveals fundamentalcharacteristics of the transaction clump tags that can be used to decodethe transaction clump tags to enforce the ordering rules. Specifically,if one or more non-posted transactions exists between a first postedtransaction and the next posted transaction, the non-posted bit of thetransaction clump tag of the next posted transaction is the inverse ofthe non-posted bit of the transaction clump tag of the first postedtransaction. Similarly, if one or more completion transactions existbetween a first posted transaction and the next posted transaction, thecompletion bit of the transaction clump tag of the next postedtransaction is the inverse of the completion bit of the transactionclump tag of the first posted transaction. Furthermore, if one or moreposted transactions exist between a first non-posted transaction and thenext non-posted transaction, the non-posted bit of the transaction clumptag of the next non-posted transaction is the inverse of the non-postedbit of the transaction clump tag of the first non-posted transaction.Similarly, if one or more posted transactions exist between a firstcompletion transaction and the next completion transaction, thecompletion bit of the transaction clump tag of the next completiontransaction is the inverse of the completion bit of the transactionclump tag of the first completion transaction.

The transition of the appropriate bits between transactions is used toimplement the ordering rules. Specifically, the transitions of thenon-posted bit of the transaction clump tags of the posted transactionsare used to increment a first counter. The first counter is decrementedby transitions of the non-posted bit of the transaction clump tags ofthe non-posted transactions. The value of the first counter along withsome other information is sufficient to implement the ordering rules fornon-posted transactions as explained in greater detail below. Similarlya second counter that is incremented by the transitions of thecompletion bit of the transaction clump tags of the posted transactionand decremented by the transitions of the completion bit of thetransaction clump tags of the completion transactions is used toimplement the ordering rules for completion transactions, as explainedin more detail below.

FIG. 7 shows one embodiment of transaction clump tag decoding unit 450having comparators 710, 720, 750 and 760, counters 730 and 770, anon-posted status control unit 740, and a non-posted status control unit780. Comparator 710 compares a previous posted transaction clump tagnon-posted bit P_P_NPB (i.e., the non-posted bit of the transactionclump tag of the previous posted transaction) with a current postedtransaction clump tag non-posted bit C_P_NPB (i.e. the non-posted bit ofthe transaction clump tag of the current posted transaction). Ifprevious posted transaction clump tag non-posted bit P_P_NPB is notequal to current posted transaction clump tag non-posted bit C_P_NPBthen comparator 710 causes counter 730 to increment by one. If previousposted transaction clump tag non-posted bit P_P_NPB is equal to currentposted transaction clump tag non-posted bit C_P_NPB then comparator 710does not cause counter 730 to increment by one. Due to the initialvalues used above with regards to the transaction clump tags, previousposted transaction clump tag non-posted bit P_P_NPB is initialized to avalue of logic 0. Previous posted transaction clump tag non-posted bitP_P_NPB is updated to equal the current posted transaction clump tagnon-posted bit C_P_NPB when the current posted transaction is processedand removed from transaction clump tag decoding unit 450. The currentposted transaction clump tag non-posted bit C_P_NPB is updated when anew current posted-transaction is brought into transaction clump tagdecoding unit 450. Counter 730 is incremented at most once for eachcurrent posted transaction.

Comparator 720 compares a previous non-posted transaction clump tagnon-posted bit P_NP_NPB (i.e., the non-posted bit of the transactionclump tag of the previous non-posted transaction) with a currentnon-posted transaction clump tag non-posted bit C_NP_NPB (i.e. thenon-posted bit of the transaction clump tag of the current non-postedtransaction). If previous non-posted transaction clump tag non-postedbit P_NP_NPB is not equal to current non-posted transaction clump tagnon-posted bit C_NP_NPB then comparator 720 causes counter 730 todecrement by one. If previous non-posted transaction clump tagnon-posted bit P_NP_NPB is equal to current non-posted transaction clumptag non-posted bit C_NP_NPB then comparator 720 does not cause counter730 to decrement by one. Due to the initial values used above withregards to the transaction clump tags, previous non-posted transactionclump tag non-posted bit P_NP_NPB is initialized to a value of logic 0.Previous non-posted transaction clump tag non-posted bit P_NP_NPB isupdated to equal the current non-posted transaction clump tag non-postedbit C_NP_NPB when the current non-posted transaction is processed andremoved from transaction clump tag decoding unit 450. Current non-postedtransaction clump tag non-posted bit C_NP_NPB is updated when a newcurrent non-posted-transaction is brought into transaction clump tagdecoding unit 450. First counter 730 is decremented at most once foreach current non-posted transaction.

Counter 730 is initialized at an initial count value ICV, which is largeenough to avoid counter 730 from being decremented to a negative value.In one embodiment of the present invention, initial count value ICV isequal to two. Counter 730 provides a non-posted count NP_CNT tonon-posted status control unit 740, which generates current non-postedtransaction blocked signal CNPT_BS. When non-posted count NP_CNT isgreater than or equal to initial count value ICV, the current non-postedtransaction is not blocked and non-posted status control unit 740 drivescurrent non-posted transaction blocked signal to a not blocked logiclevel. When non-posted count NP_CNT is equal to initial count value ICVminus one (i.e., ICV−1), and there are no received but unprocessedposted transactions, i.e. there is no current posted transaction and noposted transactions in posted transaction queue 420 as indicated byposted transaction queue empty flag PTQ_EF being at an empty logiclevel, then the current non-posted transaction is not blocked andnon-posted status control unit 740 drives current non-posted transactionblocked signal to a not blocked logic level. However, when non-postedcount NP_CNT is equal to initial count value ICV minus one (i.e.,ICV−1), and there are received but unprocessed posted transaction, i.e.posted transactions in posted transaction queue 420 as indicated byposted transaction queue empty flag being at a non-empty logic level orthere is a current posted transaction, then the current non-postedtransaction is blocked and non-posted status control unit 740 drivescurrent non-posted transaction blocked signal to a blocked logic level.When non-posted count NP_CNT is equal to initial count value ICV minustwo (i.e., ICV−2), then the current non-posted transaction is blockedand non-posted status control unit 740 drives current non-postedtransaction blocked signal to a blocked logic level. With thetransaction clump tag scheme described above, non-posted count NP_CNTshould not be less than initial count value ICV minus two, thus an errorcondition would exist if non-posted count NP_CNT is less than initialcount value ICV minus two.

Comparator 750 compares a previous posted transaction clump tagcompletion bit P_P_CB (i.e., the completion bit of the transaction clumptag of the previous posted transaction) with a current postedtransaction clump tag completion bit C_P_CB (i.e. the completion bit ofthe transaction clump tag of the current posted transaction). Ifprevious posted transaction clump tag completion bit P_P_CB is not equalto current posted transaction clump tag completion bit C_P_CB thencomparator 750 causes counter 770 to increment by one. If previousposted transaction clump tag completion bit P_P_CB is equal to currentposted transaction clump tag completion bit C_P_CB then comparator 750does not cause counter 770 to increment by one. Due to the initialvalues used above with regards to the transaction clump tags, previousposted transaction clump tag completion bit P_P_CB is initialized to avalue of logic 0. Previous posted transaction clump tag completion bitP_P_CB is updated to equal the current posted transaction clump tagcompletion bit C_P_CB when the current posted transaction is processedand removed from transaction clump tag decoding unit 450. The currentposted transaction clump tag completion bit C_P_CB is updated when a newcurrent posted-transaction is brought into transaction clump tagdecoding unit 450. Counter 770 is incremented at most once for eachcurrent posted transaction.

Comparator 760 compares a previous completion transaction clump tagcompletion bit P_C_CB (i.e., the completion bit of the transaction clumptag of the previous completion transaction) with a current completiontransaction clump tag completion bit C_C_CB (i.e. the completion bit ofthe transaction clump tag of the current completion transaction). Ifprevious completion transaction clump tag completion bit P_C_CB is notequal to current completion transaction clump tag completion bit C_C_CBthen comparator 760 causes counter 770 to decrement by one. If previouscompletion transaction clump tag completion bit P_C_CB is equal tocurrent completion transaction clump tag completion bit C_C_CB thencomparator 760 does not cause counter 770 to decrement by one. Due tothe initial values used above with regards to the transaction clumptags, previous completion transaction clump tag completion bit P_C_CB isinitialized to a value of logic 0. Previous completion transaction clumptag completion bit P_C_CB is updated to equal the current completiontransaction clump tag completion bit C_C_CB when the current completiontransaction is processed and removed from transaction clump tag decodingunit 450. Current completion transaction clump tag completion bit C_C_CBis updated when a new current completion-transaction is brought intotransaction clump tag decoding unit 450. Counter 770 is decremented atmost once for each current completion transaction.

Counter 770 is initialized at an initial count value ICV which is largeenough to avoid counter 770 from being decremented to a negative value.In one embodiment of the present invention, initial count value ICV isequal to two. Counter 770 provides a completion count C_CNT tocompletion status control unit 780, which generates current completiontransaction blocked signal CCT_BS. When completion count C_CNT isgreater than or equal to initial count value ICV, the current completiontransaction is not blocked and completion status control unit 780 drivescurrent completion transaction blocked signal CCT_BS to a not blockedlogic level. When completion count C_CNT is equal to initial count valueICV minus one (i.e., ICV−1) and there are no received but unprocessedposted transactions, i.e. there is no current posted transaction and noposted transactions are in posted transaction queue 420 as indicated byposted transaction queue empty flag PTQ_EF being at an empty logiclevel, then the current completion transaction is not blocked andcompletion status control unit 780 drives current completion transactionblocked signal to a not blocked logic level. However, when completioncount C_CNT is equal to initial count value ICV minus one (i.e., ICV−1),and there are received but unprocessed posted transaction, i.e., postedtransactions in posted transaction queue 420 as indicated by postedtransaction queue empty flag being at a non-empty logic level or thereis a current posted transaction, then the current completion transactionis blocked and completion status control unit 780 drives currentcompletion transaction blocked signal to a blocked logic level. Whencompletion count C_CNT is equal to initial count value ICV minus two(i.e., ICV−2), then the current completion transaction is blocked andcompletion status control unit 780 drives current completion transactionblocked signal to a blocked logic level. With the transaction clump tagscheme described above, completion count C_CNT should not be less thaninitial count value ICV minus two, thus an error condition would existif completion count C_CNT is less than initial count value ICV minustwo.

As stated above, some embodiments of the present invention embed thetransaction clump tags within the transactions rather than requiringadditional memory space in the transaction queues. For example, in anembodiment of the present invention for use with the PCI Express bus,the transaction clump tags are placed in two of the eight reserved bitsof the PCI transaction header. Because the PCI Express standard definesthe value of the reserved bits to be zero, transaction clump tagdecoding unit 450 resets the reserved bits that were used for thetransaction clump tag to zero as current transactions are processed.

In the various embodiments of the present invention, novel structureshave been described for enforcing ordering rules for transactions. Thevarious embodiments of the structures and methods of this invention thatare described above are illustrative only of the principles of thisinvention and are not intended to limit the scope of the invention tothe particular embodiments described. For example, in view of thisdisclosure those skilled in the art can define other ordering rules,transaction clump tags, transaction clump tagging units, transactionclump tag decoding units, non-posted status control units, completionstatus control units, and use these alternative features to create amethod, circuit, or system according to the principles of thisinvention. Thus, the invention is limited only by the following claims.

1. A method of processing a plurality of data transactions, having afirst data transaction, a second data transaction and a third datatransaction, wherein the first data transaction and the third datatransaction are of a first transaction type, the second data transactionis of a second transaction type, and wherein the first data transactionprecedes the second data transaction and the second data transactionprecedes the third data transaction, the method, comprising: assigning afirst transaction clump tag to the first data transaction, wherein thefirst transaction clump tag comprises a second-transaction-type bit;assigning a second transaction clump tag to the second data transaction,wherein the second transaction clump tag comprises asecond-transaction-type bit set to an inverse of thesecond-transaction-type bit of the first transaction clump tag; andassigning a third transaction clump tag to the third data transaction,wherein the third transaction clump tag comprises asecond-transaction-type bit set to an inverse of the second-transactiontype bit of the second transaction clump tag.
 2. The method of claim 1,further comprising: storing the first data transaction in afirst-transaction-type queue; storing the second data transaction in asecond-transaction-type queue; and storing the third data transaction inthe first-transaction-type queue.
 3. The method of claim 1, wherein theplurality of data transaction includes a fourth data transaction andwherein the fourth data transaction is of a third transaction type, thefourth data transaction follows the first data transaction, and thefourth data transaction precedes the second data transaction.
 4. Themethod of claim 3, wherein the first transaction clump tag comprises athird-transaction-type bit and the third transaction clump tag comprisesa third-transaction type bit.
 5. The method of claim 4, furthercomprising assigning a fourth transaction clump tag to the fourth datatransaction, wherein the fourth transaction clump tag comprises athird-transaction-type bit set to an inverse of thethird-transaction-type bit of the first transaction clump tag; andwherein the third transaction clump tag further comprises athird-transaction-type bit set to an inverse of the third-transactiontype bit of the fourth transaction clump tag.
 6. The method of claim 3,further comprising: storing the first data transaction in afirst-transaction-type queue; storing the second data transaction in asecond-transaction-type queue; storing the third data transaction in thefirst-transaction-type queue; and storing the fourth data transaction ina third-transaction-type queue.
 7. The method of claim 1, wherein theplurality of data transaction includes a fourth data transaction andwherein the fourth data transaction is of the second transaction type,the fourth data transaction follows the first data transaction, and thefourth data transaction precedes the second data transaction.
 8. Themethod of claim 7, further comprising assigning a fourth transactionclump tag to the fourth data transaction, wherein the fourth transactionclump tag comprises a second-transaction-type bit set to an inverse ofthe second-transaction-type bit of the first transaction clump tag. 9.The method of claim 1, further comprising assigning a fourth transactionclump tag to a fourth data transaction wherein the fourth datatransaction is of the first transaction type and the fourth datatransaction follows the third data transaction and wherein the fourthtransaction clump tag comprises a second-transaction-type bit set to aninverse of the second-transaction-type bit of the second transactionclump tag.
 10. The method of claim 1, further comprising storing thefirst transaction clump tag within the first data transaction; storingthe second transaction clump tag within the second data transaction; andstoring the third transaction clump tag within the third datatransaction.
 11. The method of claim 10, wherein the storing the firsttransaction clump tag within the first data transaction, furthercomprising replacing one or more reserved bits of the first datatransaction with the first transaction clump tag.
 12. The method ofclaim 11, further comprising resetting the reserved bits of the firstdata transaction.
 13. The method of claim 1, wherein the first datatransaction is a posted transaction.
 14. The method of claim 1, whereinthe second data transaction is a non-posted transaction.
 15. The methodof claim 1, wherein the second data transaction is a completiontransaction.
 16. A data system for processing a plurality of datatransactions comprising a transaction clump tagging unit coupled toreceive the plurality of data transaction and configured to assign atransaction clump tag to each data transaction; a first transaction typequeue coupled to receive data transactions of a first transaction typefrom the transaction clump tagging unit; and a second transaction typequeue coupled to receive data transaction of a second transaction typefrom the transaction clump tagging unit.
 17. The data system of claim16, wherein the transaction clump tagging unit is configured to assign atransaction clump tag to a data transaction of a first transaction typeto have a second-transaction-type bit set to an inverse of asecond-transaction type bit of a transaction clump tag assigned to amost recent data transaction of the second transaction type.
 18. Thedata system of claim 17, wherein the transaction clump tagging unit isconfigured to assign a transaction clump tag to a data transaction of asecond transaction type to have a second-transaction-type bit set to aninverse of a second-transaction type bit of a transaction clump tagassigned to a most recent data transaction of a first transaction type.19. The data system of claim 16, further comprising a third transactiontype queue coupled to receive data transaction of a third transactiontype from the transaction clump tagging unit.
 20. The data system ofclaim 19, wherein the transaction clump tagging unit is configured toassign a transaction clump tag to a data transaction of a firsttransaction type to have a third-transaction-type bit set to an inverseof a third-transaction type bit of a transaction clump tag assigned to amost recent data transaction of the third transaction type.
 21. The datasystem of claim 20, wherein the transaction clump tagging unit isconfigured to assign a transaction clump tag to a data transaction of athird transaction type to have a third-transaction-type bit set to aninverse of a third-transaction type bit of a transaction clump tagassigned to a most recent data transaction of a first transaction type.22. A data system for processing a plurality of data transactions,having a first data transaction, a second data transaction and a thirddata transaction, wherein the first data transaction and the third datatransaction are of a first transaction type, the second data transactionis of a second transaction type, and wherein the first data transactionprecedes the second data transaction and the second data transactionprecedes the third data transaction, the data system, comprising: meansfor assigning a first transaction clump tag to the first datatransaction, wherein the first transaction clump tag comprises asecond-transaction-type bit; means for assigning a second transactionclump tag to the second data transaction, wherein the second transactionclump tag comprises a second-transaction-type bit set to an inverse ofthe second-transaction-type bit of the first transaction clump tag; andmeans for assigning a third transaction clump tag to the third datatransaction, wherein the third transaction clump tag comprises asecond-transaction-type bit set to an inverse of the second-transactiontype bit of the second transaction clump tag.
 23. The data system ofclaim 22, further comprising: means for storing the first datatransaction in a first-transaction-type queue; means for storing thesecond data transaction in a second-transaction-type queue; and meansfor storing the third data transaction in the first-transaction-typequeue.
 24. The data system of claim 22, wherein the plurality of datatransaction includes a fourth data transaction and wherein the fourthdata transaction is of a third transaction type, the fourth datatransaction follows the first data transaction, and the fourth datatransaction precedes the second data transaction.
 25. The data system ofclaim 24, wherein the first transaction clump tag comprises athird-transaction-type bit and the third transaction clump tag comprisesa third-transaction type bit.
 26. The data system of claim 36, furthercomprising means for assigning a fourth transaction clump tag to thefourth data transaction, wherein the fourth transaction clump tagcomprises a third-transaction-type bit set to an inverse of thethird-transaction-type bit of the first transaction clump tag; andwherein the third transaction clump tag further comprises athird-transaction-type bit set to an inverse of the third-transactiontype bit of the fourth transaction clump tag.
 27. The data system ofclaim 24, further comprising: means for storing the first datatransaction in a first-transaction-type queue; means for storing thesecond data transaction in a second-transaction-type queue; means forstoring the third data transaction in the first-transaction-type queue;and means for storing the fourth data transaction in athird-transaction-type queue.
 28. The data system of claim 22, whereinthe plurality of data transaction includes a fourth data transaction andwherein the fourth data transaction is of the second transaction type,the fourth data transaction follows the first data transaction, and thefourth data transaction precedes the second data transaction.
 29. Thedata system of claim 28, further comprising means for assigning a fourthtransaction clump tag to the fourth data transaction, wherein the fourthtransaction clump tag comprises a second-transaction-type bit set to aninverse of the second-transaction-type bit of the first transactionclump tag.
 30. The data system of claim 22, further comprising means forassigning a fourth transaction clump tag to a fourth data transactionwherein the fourth data transaction is of the first transaction type andthe fourth data transaction follows the third data transaction andwherein the fourth transaction clump tag comprises asecond-transaction-type bit set to an inverse of thesecond-transaction-type bit of the second transaction clump tag.
 31. Thedata system of claim 22, further comprising means for storing the firsttransaction clump tag within the first data transaction; means forstoring the second transaction clump tag within the second datatransaction; and means for storing the third transaction clump tagwithin the third data transaction.
 32. The data system of claim 31,wherein the means for storing the first transaction clump tag within thefirst data transaction, further comprising means for replacing one ormore reserved bits of the first data transaction with the firsttransaction clump tag.
 33. The data system of claim 32, furthercomprising means for resetting the reserved bits of the first datatransaction.
 34. The data system of claim 22, wherein the first datatransaction is a posted transaction.
 35. The data system of claim 22,wherein the second data transaction is a non-posted transaction.
 36. Thedata system of claim 22, wherein the second data transaction is acompletion transaction.